The general hypothesis is that the decrease in muscle mass and function that occurs in elderly individuals can be ameliorated with an appropriate nutritional supplementation. The goal is to develop a scientific basis for a nutritional supplement that will stimulate net muscle anabolism to a greater extent than a comparable amount of their normal diet. These investigators will determine the mixture of a given amount of amino acids that will stimulate muscle protein synthesis. The effect of non-protein energy alone and in conjunction with amino acid intake, will also be evaluated. Studies will be performed in a group of healthy, active elderly subjects and in a separate group of frail elderly subjects. These studies will provide the metabolic basis for the formulation of a nutritional supplement that should optimally stimulate net muscle protein synthesis.